Redirects with mod_rewrite

Overview

A redirect changes the URL, in browser, from one URL to another. A variety of redirect codes exist to force a variety of behaviors in the browser (or spider).

Usage

All redirects are controlled through a .htaccess file in the document root of your target domain or subdomain. The following stanza is a common, and simple redirect if the URL is www.example.com, then redirect to example.com:

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.example\.com$
RewriteRule ^(.*)$ http://example.com/$1 [R,L]

Likewise, a converse rule to redirect example.com to www.example.com would be a minor alteration of RewriteCond and RewriteRule:

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^example\.com$
RewriteRule ^(.*)$ http://www.example.com/$1 [R,L]

Notice the difference? RewriteCond stands for “rewrite condition” and RewriteRule “rewrite rule”. That is to say, if this condition matches, apply this rule, which consists of a regular expression substitution, to the current URL. Rewrites operate using regular expressions, in particular Perl-compatible (“PCRE”).

Required components

A redirect always requires two crucial components:

RewriteEngine On
RewriteBase /

RewriteEngine On enables URL rewriting. RewriteBase / anchors all rules to the current directory. [su_highlight]RewriteBase is absolutely necessary when used with addon domains and subdomains.[/su_highlight]

The following examples will omit these 2 lines for brevity:

Examples

Redirect to SSL

RewriteCond %{HTTPS} !^on$
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R,L]

Discussion: look for an environment variable called HTTPS. Whenever SSL is used, this variable is set by the web server. If no flag is set, assume normal HTTP traffic. Redirect to SSL (https) and keep the hostname (%{HTTP_HOST}) + URL path ($1, captured from left-side)

Redirect codes

An optional number may be added onto a R flag to specify a redirect code. By default a 302 is sent in a redirect response. The following redirect codes may be used by changing R to R=XXX, where XXX is one of the following codes:

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See also

• Introduction to regular expressions and mod_rewrite (httpd.apache.org)
• URL rewriting with mod_rewrite (httpd.apache.org)